Move Comes After Antigovernment Insurgents Captured Key Areas in the North

KAMPALA Uganda—South Sudan's president has shaken up the country's military leadership, as stinging defeats by rebels and a massacre of civilians further imperil the stability of the world's youngest nation.

President Salva Kiir's decision late Wednesday to replace the army's chief of staff, Gen. James Hoth Mai, was a "routine redeployment" aimed at improving the efficiency of the military, said presidential spokesman Ateny...